Many devices such as electric alarm clocks or cordless telephones incorporate a backup battery to provide power to the device. Because the backup battery provides power when the primary source of power is interrupted, the backup battery would become discharged if it were installed prior to shipping. Presently, such a device is shipped to a purchaser without a backup battery installed, if a battery is included with the device. The installation of a backup battery is considered an inconvenience for the purchaser of the device. In particular, tools and some time may be required to install the backup battery, depending on where the battery is located in the device.
Generally, circuits have been developed to provide backup battery power after the loss of primary power. For example, U.S. Pat. No. 5,057,697 to Hammond et al. discloses an uninterruptible power supply that incorporates switching circuitry permitting a path for DC backup power to be supplied to a voltage-declining primary power supply. A first switch SW1 couples primary power to a load. A second switch SW2 couples a back-up battery to a power transition switch circuit 114 comprising a bank of zener diodes coupled in series with a pair of parallel transistors 124 which are supplied with a separate 12V DC voltage source. At a point where the backup battery no longer provides useful power, disabling circuitry disconnects the battery from the load.
Further, circuits have been developed which will close a first switch in response to detecting the application of power, followed by closing a second switch in response to detecting the loss of power. Both switches will be opened after a predetermined period of time to conserve battery power. Referring particularly to U.S. Pat. No. 4,528,459 to Wiegel, whenever the supply of primary power is detected, a first switch 18 is closed. Closing relay contact 18 couples the backup power source 16 to the series circuit comprising contact 18 and electronic switch 22, but does not connect power from the backup source to the load 12 until the second switch, electronic switch 22, is enabled. Electronic switch 22 is enabled upon decay or turn-off of primary voltage.
Accordingly, there is a need for a simple method and circuit to prevent the discharge of a backup battery which is installed prior to an initial application of primary power.